Plural-phase controlled rectifier systems
insensitive to line phase rotation



Nov. 4, 1969 w. sp Re. 26,708

PLURAL-PHASE CONTROLLED RECTIFIER SYSTEMS INSENSITIVE TO LINE PHASE ROTATION Original Filed Dec. 551, 1962 2 Sheets-Sheet 1 Nov. 4, 1969 R. w. SPINK Re. 26,708

PLURAL-PHASE CONTROLLED RECTIFIER SYSTEMS INSENSITIVE TO LINE PHASE ROTATION 2 Sheets-Sheet 2 Original Filed Dec. 31, 1962 L/ L2 L3 3 LINE a v01. TA as u/vs u VOLTAGE RELATIVE TO u/ves L2 g. 1.3

TRANSFORMER 56' C ONDAEV S/ VOL TA GE PE L A 7'! V5 TO EMITTEE OF T2 TRANS/870E T2 BA 55 TO EMITTEE VOLTAGE CAP/4C I TOE CI VOL TAGE MIN. TURN ON FOI? NEAR 4 ZERO OUTPUT VOLTAGE- l65 TURN ON FOR 97% OF MAX. ourpur I l I I l 1 United States Patent Ofiicc Re. 26,708 Reissued Nov. 4, 1969 26,708 PLURAL-PI-IASE CONTROLLED RECTIFIER SYSTEMS lNSENSlTlVE T LINE PHASE ROTATION Robert W. Splnlr, Wauwatosa, Win, or to Cutler- Hammer, luc., Milwaukee, Win, a corporation of Delaware Original No. 3,281,645, dated Oct. 25, 1966, Ser. No. 248,314, Dec. 31, 1962. Application for reissue Jan. 23, 1968, Ser. No. 702,144

Int. Cl. H0211: 7/44. 7/68 US. Cl. 321- 5 Claims Matter enclosed in heavy brackets [1 appears In the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE i A three-phase solid state rectifier bridge systerit supplied from a three-phase A.C. source for controlling a D.C. load and including series-connected controlled rectifiers in the bridge circuit and firing circuits phase-synchronized with their anode voltages to apply simultaneous fast rise time firing pulses thereto and to render such system insensitive to line phase rotation. Transistor switching circuits limit firing pulses to positive anode voltage halfcycles and RC voltage stretcher; ofiord full 180 degree firing.

C R OSS-REF EREN CE TO RELA TED A PPLICA TION This is a division of my reissue application Ser. No. 702,145 filed concurrently herewith.

BACKGROUND OF THE INVENTION This invention relates to controlled-rectifier systems and more particularly to firing control circuits for controlled-rectifiers.

While not limited thereto, the invention is especially applicable to a three-phase rectifier bridge network supplying an inductive direct current load and having controlled-rectifiers in series connection in certain branches of such bridge network to provide simultaneous very fast rise time and narrow width gate pulses successfully to fire scrie5-counected controlled-rectifiers with damaging voltage transient limitations and to prevent damaging the controlled-rcctifiers by gating thereof while load countervoltage is being applied reversely thereto.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved controlled-rectifier system.

A more specific object of the invention is to provide improved firing control circuits for controlled-rectifier networks.

Another specific object of the invention is to provide improved fast-firing control circuits for plural-phase controlled-rectifier [bridge] bridge networks.

Other objects and advantages of the invention will hereinaftcr appear.

According to the invention, there is provided a threephase rectifier bridge network energized from a threephase alternating current power supply source and being controllable to supply adjustable unidirectional electrical energy to a direct current load device such as the armature winding of a direct curcnt motor. The rectifier bridge network is of the static solid element type. While the arrangement of solid elements in the rectifier network may take various forms, it is preferably of the type having at least two series-connected controlledrectiflers in each of three branches and having one or more diodes in each of the other three branches thereof. Each of these controllcd-rectificrs is provided with an anode and a cathode forming a main current eonduction path therethrough and a gate forming with the cathode a control current conduction path whereby the firing point of the controlled-rectifier is controlled. While controlled-rectifier firing control circuits have been known heretofore, these known circuits have the disadvantages thnt they permit time lag in the firing of one series-connected controlled-rectifier relative to the firing of the other, and the firing pulse increases leakage current ,when reverse voltage is applied. A feature of the invention resides in the provision of firing control circuits which overcome such disadvantage by affording simultaneous very fast rise time narrow width gate pulses whereby simultaneously to fire series-connected controlled-rectifiers. Three such firing control circuits are provided for three respective branches of the rectifier bridge. These firing control circuits are supplied from a common voltage source and their input control terminals are connected together in parallel. Each such firing control circuit comprises a very fast acting pilot controlledrectifier which, when fired, energizes a pulse transformer. The latter is provided with a plurality of secondary windings connected to apply steep wave front and very short time firing pulses to the gate-cathode junctions of the respective series controlled-rectifiers. An input transistor provides capacitor charging current proportional to the input control voltage which in turn controls a uniiunction transistor to trigger the pilot controlledrectifier. A switching transistor is employed to establish the charging intervals of the integrating capacitor. The switching transistor is provided with a delay circuit or RC circuit to stretch its cutoff period a minimum of 30 electrical degrees whereby to afford firing control to the extreme end of the 240 electrical degree period during which forward voltage is applied to the series controlled-rectifiers. A three-phase transformer is provided to synchronize the switching transistor control voltage wave with the forward voltage wave applied to the series controlledrectifiers. Other features of the invention reside in the incorporation of means to render the system insensitive to circuit transients, to line voltage disturbances, and to line phase rotation; to afford high gain at low output, to afford low power consumption, to afford constant amplitude firing pulses over a complete electrical degree control range, to atiord simultaneous one microsecond rise time pulses to the series controlled-rectifiers and to electrically isolate the input signal from the power supply lines and the rectifier network.

BRIEF DESCRIPTION OF THE DRAWING These and other objects and advantages of the invention and the manner of obtaining them will best be understood by reference to the following detailed description of an embodiment of the invention taken in conjunction with the accompaning drawings, wherein:

FIGURE 1 diagrammatically depicts a controlled-rectifier system and firing control circuits therefor constructed in accordance with the invention, and

FIG. 2 graphically depicts operating characteristics of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there are shown power supply lines L1, L2 and L3 supplied from a three-phase alternating curent power supply source and being connected to the three input terminals of a three-phase rectifier bridge network indicated generally as RN. The positive and negative output terminals of network RN are connected across a direct current load device L. The input terminals to which lines L1, L2 and L3 are connected are in turn connected through gating type power controlling devices such as pairs of controlled-rcctifiers l, 2 and 3, respectively, to the positive output terminal of network RN. The negative output terminal of network RN is connected through pairs of unidirectionally conducting diodes 4, and 6 to the input terminals to which respective lines L1, L2 and U are connected.

Three firing control circuits 8, l0 and 12 are provided for pairs of controlled-rectifiers 1. 2 and 3, respectively. Since firing control circuits l0 and 12 are identical to firing control circuit 8, only the latter has been shown in detail to avoid complicating the drawing.

A group of three transformers PT is provided to afford supply and control voltage to the three firing control circuits Transformers PT are provided with a set of primary windings and two sets of secondary windings. Primary windings P1, P2 and P3 are connected in star relation such that first ends thereof are connected to a common neutral point and the other ends thereof are connected to lines L1. L2 and L3, respectively. The first set of secondary windings S1, S2 and 83 are magnetically coupled to primary windings Pl, P2 and P3, respectively, and are individually connected to firing control circuits 8, and 12. respectively, to supply synchronized control voltages thereto as hereinafter more fully described. The second set of secondary windings S4, S5 and S6 are connected to one another in delta relation and are magnetically coupled to primary windings P1, P2 and P3 respectively. These delta-connected secondary windings serve to stabilize the neutral connection of the primary windings and thereby prevent distortion of the voltage sine waves induced in the first set of secondary windings S1, S2 and S3. The three junctions of secondary windings S4, S5 and S6 are connected, respectively, to the three input terminals of a three-phase rectifier bridge RB. The positive and negative output terminals of the latter are connected to a voltage regulator VR. The positive and negative output terminals of the voltage regulator are connected to common supply conductors l4 and 16, respectively, whereby the supply regulated unidirectional opt-rating voltage to the three firing control circuits.

A pair of input terminals 18 are provided for applying a direct current input signal to the three firing control circuits. The positive input terminal is connected to common conductor 14 and the negative input terminal is connected to a common conductor 20 which is connected to the three firing control circuits in parallel. A resistor R1 and a unidirectional current conducting diode D1 are connected in series in that order from conductor l4 to conductor 16. The junction of resistor R1 and diode D1 is connected in parallel to firing control circuits Fl. 18 and 12 for purposes hereinafter described.

Firing control circuit 8 is provided with means for developing a linearly increasing triggering voltage. This means comprises a constant current conducting semiconductor device such as an input transistor T1 for controlling current flow to an electrical energy storage device such as a capacitor C1. Transistor T1 is of the P-N-P type and its emitter is connected through a resistor R2 to conductor 14 and through the latter to the positive input voltage terminal. The base of transistor T1 is con nected to conductor 20 and through the latter to the negative input voltage terminal. The collector of transistor T1 is connected through capacitor C1 to common conductor 16 and through the latter to the negative output terminal of voltage regulator VR.

Firing control circuit 8 is also provided with switching means for determining the charging periods of capacitor C1 in synchronism with the periods during which forward voltage is applied from the power supply lines to controlled rectifiers 1 in the associated branch of rectificr bridge network RN. This means comprises a switching or resetting transistor T2 of the P-N-P type having its emitter connected to the junction between capacitor C1 and the collector of transistor T1 and having its collector connected to the other side of capacitor C1. This means also comprises a control voltage source for transistor T2 including primary winding P1 connected b tween power supply line LI and the transformer neutral point and secondary winding S1 coupled to primary wind ing Pl. Secondary winding S1 provides a degree voltage half-cycle in synchronism with the 240 dog-re" forward voltage applied to controlled rcctifiers 1, as he". inafter more fully described, and because its phase rr'n' .tionship is such that its positive voltage starts 30 degrees after the start of the forward voltage and terminates i degrees before the end of the forward voltage, the system is insensitive to phase rotation. Because this positive voltage starts 30 degrees ahead of the earliest needed firing pulse for maximum output, complete required 180 degree firing control can be obtained by stretching the trailing end of this positive voltage half-cycle by 30 mo e degrees. This stretching means comprises a unidirectional diode D2 and an RC circuit comprising a capacitor C2 and a resistor R3. One end of secondary winding S1 is connected to the emitter of transistor T2 and the other end thereof is connected through diode D2 in its forward, low impedance direction to the base of transister T2. Resistor R3 is connected across diode D2. Capacitor C2 is connected between the emitter and base of transistor T2.

Firing control circuit 8 is further provided with means for applying firing pulses to the gates of controlled rectifiers l and triggering means for such firing pulse applying means under the control of the aforementioned linear voltage developing means.

This firing pulse applying means comprises a very fast acting gating type pilot device or pilot controlled-rectifier SCR and a pulse transformer PLT. Controlled-rectifier SCR may be a silicon controlled-rectifier and may be similar to controlled rectifiers l, 2 and 3 in network RN except that it is smaller and has a much smaller voltage and current rating. The anode of controlled-rectifier SCR is connected through primary winding P of pulse transformer PLT to the junction between capacitor C1 and the collector of transistor T1. The cathode of controlled-rectifier SCR is connected to the common junction between resistor R1 and diode D1. The gate of controlledrectifier SCR is connected through a resistor R4 to the junction between capacitor C1 and common conductor 16. Secondary winding SW1 of transformer PLT is connected to the gate and cathode of the first controlled-rectifier of pair 1 thereof and secondary winding SW2 of transformer FLT is connected to the gate and cathode of the second controlled-rectifier of such pair in like manner.

The aforementioned triggering means comprises a uni junction transistor UT arranged to respond to the voltage on capacitor C1 to trigger or fire controlled-rectifier SCR. Base B2 is connected through a resistor R5 to positive voltage conductor 14 and base B1 is connected through the aforementioned resistor R4 to negative voltage conductor 16 whereby to supply regulated inter-base voltage to the unijunction transistor. The junction between capacitor C1 and the collector of transistor T1 is connected through a resistor R6 to the emitter of unijunction 1transistor UT whereby to apply control voltage to the atter.

Although the details of firing control circuits 10 and 12 have not been shown since they are similar to the details of firing control circuit 8 just described, the external connections thereto are the same as shown in FIG. 1. That is, positive voltage conductor 14 and negative voltage conductor 16 are connected to each of circuits 10 and 12 in the sum manner as described in connection with circuit 8. Negative input signal voltage conductor 20 is connected in the same manner to the base of a similar input transistor in each of circuits l0 and 12. The junction between resistor R1 and diode D1 is connected through a conductor 22 to the cathode of a controlledrectifier similar to controlled-rectifier in each of circuits 10 and 12. Secondary windings S2- and 53 are connected through pairs of conductors i l and 25 to circuits 1!) and 12 in the same manner as secondary winding S1 connected to circuit 8. Moreover, the pairs of secondary windings of pulse tranhiorrncrs in circuits 10 and 12 similar to pulse trans ormer PEST in circuit 8 are connected in like manner iii! oi tour conductors in and 39 to pairs 2 and FE at c: olled-rcctilfiers in the other two controlled hrarichcz of hririge network RN, respcctivcly.

voltage regulator "R shown schematically in the lower left-hand portion of FIG. 1 is provided to afford a D.C. voltage of constant magnitude to common conductors 14 and 16 and through the latter to firing control circuits ii, it; 12. Voltage regulato s of this type are well and available on the File! t. While voltage regutlur type may take y mas forms, a preferred i. l. is this system the Cutler-Hammer Reference Regulator Module For example, if three-phase delta-connected secondary windings S4, S5 and S6 provide a voltage such that full-wave rectification thereof by bridge RB provides 37 to 45 volts D.C. to regulator VR, the latter provides 35 volts regulated D.C. to conductor! 4 and 3.6.

it suitable source of input signal voltage may be connected to input terminals 18. This source may consist of any known D.C. voltage source having a selectively and continuously adjustable magnitude sufficient to control current conduction in transistor T1 or may be obtained from voltage regulator VR through a continuously adjustable potentiometer or the like.

Secondary windings SW1 and SW2 of pulse transformer PLT are wound of resistance wire to provide each of these windings with 5 to 6 ohms of resistance which functions to limit the gate currents of controlled rectifiers 1. This secondary resistance is desired because the impedance of controlled rectifier SCR when conducting is very small 2, therefore, the circuit of primary winding P does not w enough impedance to l mit such gate current when capacitor C1 is discharged thercthrough.

The operation of the system of FIG. 1 will now be LW'H ltfld with refer nce to the curves in FIG. 2 showing op rating characteristics thereof. Since firing control circults l and 12 operate controllerl-rectifier pairs 2 and 3 in the same manner as firing control circuit 8 operates routrollcd'rcctificr pair 1, only the latter operation will be rl z ribed in detail. It will be apparent that, depending on the phase sequence of the AC. power supply, one order of conduction will he controiled rectifier pairs 1, 2 and 3 in sequence in that order whcreufter the conduction would be similarly repeated. That is, current will flow thm'mgh controlled-rectifier pair 1 and load L and then lrsi through diodes and than through diodes 6. When controllcd-rcctificrs 2 are fired next in the sequence, current will now thcrethrough and through the load and then first through diodes 6 and then through diodes 4. When controlled-rcctificrs 3 are fired last in the sequence, current will flow therethrough and through the load and then first through diodes 4 and then through diodes 5. This order of current flow is illustrated in FIGS. 2(a) and (b) wherein line L1 is shown as being first positive relative to line L1 and being then positive relative to line L3. Although not shown in FIG. 2(b) to avoid complicating the curves therein, it is apparent in FIG. 2(a) that line L2 is first positive relative to line L3 and is then positive relative to line LI. Next in sequence, line L3 becomes positive first with respect to line L1 and then becomes positive with respect to line L2. Thereafter this same sequence is repeated.

Referring to FIG. 1. it will be apparent that a forward voltage is applied to controlled-rcctificrs 1 for a 240 electrical degree period according to the curve in FIG. 2(b) from lines L1, L2 and L3. The line voltage is also applied 1 as shown in FIG. 2th). Due to the slut COELTECL. the transformer primary windings, the vol age jfiliilljllt in in secondary winding 51 persists before reversal tur electrical degrees, that is, it lags the forward with of the associated controllcd-rectifiers by 30 degrees a decreases to zero 36 degrees ahead of the forward voltage as shown in FIGS. 203) and (c). In other words. the voltage wave in FIG. 2(a) is in phase with t e m c l ll 'l age wave L1 in FIG. 2(a). Similar volta e in secondary windings S2 and S3 in synchrt the forward voltages on controllcdrcctificr pairs. 2. .mo respectively, in sequence.

The aforementioned energization of the primary windings of transformers PT also causes energization of the second set of delta-connected secondary windings S4, S5 and S6. As a result, current fiows therefrom through rectifier bridge RB to voltage regulator VR. In turn. a regulated D.C. voltage is applied from the positivc a negative output terminals of the voltage regulator to cor:- doctors 14 and 16. This causes current flow through rcaistor R1 and diode D1. The forward voltage drop of diode D1 is about one-half volt and is sufficient to nullify any voltage cfiect on the gate-cathode junction of contro l rectifier SCR which might be caused by interbase urrvn flow through unijunction transistor UT and resistor l'r'i. That is, any tendency on the part of such intcrhase CUT" rent to trigger controlled-rectifier SCR into its conducting state will be oflset by the forward voltage drop across diode D1 and the latter will bias the gate of controlled rectifier SCR slightly negative relative to the cathode thereof to maintain the same non-conducting.

Interbase voltage for urtijunction transistor HT sup plied from conductors i4 and 16 through resistors $5 R4. Operating voltage is also supplied from conductors 14 and 16 through resistor R2 and capacitor C1 to the emitter-collector junction of transistor T1.

Application of control input voltage to input tcrrr 18 of the polarity shown thcrcat renders the nose of trt. 1- sistor T1 negative relative to the emitter thereof. is a result, transistor T1 is rendered conducting an: tut from conductor 14 through resistor R2 and the c unch collector junction of transistor T1. This current hen flows either through switching transistor T2 when it is rendered conducting or through capacitor C1 to charge the latter when transistor T2 is rendered now-conducting as hereinafter more fully described.

The aforementioned voltage on transformer secondary winding S1, that is, the positive half-cycle thereof shown in FIG. 2(c) causes current flow through diode D2 in its forward, low impedance direction to charge capacitor C2 to the polarity depicted thereat. This charge on capacitor C2 renders the base of switching transistor T2 positive relative to the emitter thereof to render the switching transistor non-conducting. As a result, instead of flowing through transistor T2, the aforementioned current flowing through input transistor T1 now charges capacitor C1 linearly as shown by the upward slope of the curve in FIG. 2(e). The charging of capacitor C1 is linear because current of constant magnitude flows through input transistor T1. The slope of this charging curve, that is, the charging rate, can be increased by increasing the magnitude of the input signal voltage at terminals 18. This slope can be increased from the minimum shown in FIG. 2(e) past that shown in FIG. 2(f) to a maximum hereinafter described when the system is adjusted to maximum power output to the load.

The charge on capacitor C1 controls application of firing pulses to controlled rectifiers 1. When the voltage on capacitor C1 increases to a predetermined magnitude. for example, volts, which voltage is applied through resistors R6 and R4 to the emitter-base Bl junction of unijunction transistor UT, it renders the unijunction transistor conducting. As a result, capacitor C1 starts to discharge and causes current flow through resistor R6, the emitter-base B1 junction of the unijunction transistor and resistor R4. The voltage drop on resistor R4 is of opposite polarity to the forward voltage drop on diode D1, overcomes such forward voltage drop and renders the gate of controlled-rectifier SCR positive relative to the cathode thereof to cause current flow through the gate-cathode junction and diode D1 to trigger this pilot controlled-rectifier into its conducting state. Capacitor Cl then quickly discharges completely through primary winding P of pulse transformer PLT, the anode-cathode junction of controlled-rectifier SCR and diode D1. This causes very fast rise time, I microsecond, narrow width gate pulses to be applied simultaneously from secondary windings SW1 and SW2 to the gate-cathode junctions of controlled-rectifiers 1. These gate pulses fine the controlled-rectifiers to cause current flow therethrough to load L for the remainder of their 240 electrical degree forward voltage period shown in FIGURE 2(b). These gate pulses have a very fast rise time due to the rapidity with which controlled-rectifier SCR can be triggered, and have a very narrow pulse width or short duration of about $0 microseconds due to the rapidity with which controlled-rectifier SCR discharges capacitor C1.

The RC circuit comprising resistor R3 and capacitor C2 is provided to afford adjustment of the firing angle of controlled-rectifiers 1 to the extreme end of their forward voltage wave shown in FIG. 2(b). Since the voltage on transformer secondary winding S1 reverse polarity as shown in FIG. 2(c) degrees before the end of the controlled-rectifier forward voltage shown in FIG. 2(b), this reversal would cause switching transistor T2 to conduct and would terminate charging of capacitor C1 unless some means are provided to modify this action. For this purpose, the RC circuit is provided. When the voltage of secondary winding 81 decreases through zero to a. negative value as shown in FIG. 2(c), capacitor C2 discharges through resistor R3. However, while capacitor C2 is discharging, the voltage thereon maintains transistor T2 non-conducting for at least an additional 30 electrical degrees. In other words, capacitor C2 and resistor R3 stretch the trailing end of the positive halfcycle of voltage from that shown in FIG. 2(c) to that shown in FIG. 2(d). Consequently, when the input signal is adjusted to its minimum magnitude, this stretched voltage will maintain transistor T2 non-conducting for an additional 30 degrees whereby voltage integrating capacitor C1 will continue to charge to the extreme end of the forward voltage on controlled-rectifiers 1 before the voltage on capacitor C1 reaches a value sufficient to trigger the unijunction transistor. In this manner, by adjusting the input control voltage, the firing angle of controlledrectifiers 1 can be adjusted anywhere from zero load current to a full ISO-degree firing angle because 30 degrees is the minimum lag from the start of charging of capacitor C1 required for firing of controlled-rectifiers 1. A substantially I65-degree firing angle is approximately 97 percent of maximum load current is depicted in FIG. 2(f). As shown in FIG. 2(f), at a large value of output to the load, capacitor C1 charges to a triggering value at 75 degrees, that is, 240 minus 75 equaling a firing angle of 165 degrees. After capacitor C1 discharges and fires controlled-rectifiers 1, the latter continue to conduct-for the remainder of their forward voltage period. As shown in FIG. 2), capacitor C1 recharges and discharges a number of times during the remainder of such 240 degree forward voltage period. However, such recharging will have no unwanted effect on controlled-rectifiers 1 as they have already been fired.

During the succeeding negative half-cycle of the voltage of transformer secondary winding 81 depicted in FIG. 2(c), capacitor C2 discharges completely as shown in FIG. 2(d). At the end of such complete discharge the voltage reverses polarity whereby the emitter of transistor T2 is rendered positive relative to the base thereof. Thus, the current flowing through resistor R2 and transistor T1 is diverted through transistor T2 to prevent charging of capacitor C1 until 30 degrees after forward voltage is again applied to controlled-rectifiers 1 when transistor T2 again resets the charging of capacitor C1 In a manner similar to that just described, input control voltage is applied to firing control circuits It and 12. Also, control voltages are applied thereto from secondary windings S2. and S3 to operate firing control circuits Ill and 12 in sequence after each operation of circuit 8. In this manner, controlled-rectifier pairs 1, 2 and 3 are fired in sequence to energize the load.

Resistor R5 in FIG. 1 provides temperature compensation and limits the wattage dissipation in unijunction transistor UT to prevent thermal runaway. Resistore R6 limits the voltage that can be applied to the gate of controlled-rectifier SCR to less than its maximum voltage rating. Since the emitter-base B1 impedance of the unijunction transistor decreases to a small value when conducting, the voltage of capacitor C1 is applied across resistor R6 and R4 [is] in series. These resistors function as a voltage divider to limit the maximum gate voltage of controlled-rectifier SCR.

The system shown in FIG. 1 incorporates a plurality of other features hereinbefore mentioned. The system is insensitive to electrical transients because they are averaged when capacitor C1 is charged, that is, when capacitor C1 integrates the current with respect to time flowing through input transistor T1. The application of a regulated direct current voltage supply to conductors 14 and 16 readers the system insensitive to line voltage disturbances. Capacitor C2 absorbs transients on transformer secondary winding S1 when such secondary winding voltage goes through zero value whereby such transients will not affect the starting point of charging of capacitor C1. The system provides high gain at the low power end of its adjustable range of operation. This high gain is obtained by a non-linear characteristic; that is, the charging time of capacitor C1 is inversely proportional to the value of charging current to afford a hyperbolic gain curve for a range of values of input signal voltage. The firing control circuits consume a relatively small amount of power; that is, only a little over one watt of power. easily controls a -horsepower load when the latter is the armature winding of a direct current shunt-wound motor. Due to the connection of the primary windings of transfonrners PT in star relation whereby line to neutral voltage, FIG. 2(c), becomes available for controlling switching transistor T2, the system is rendered independent of supply voltage phase rotation. That is, if the three-phase power connections to lines L1, L2 and 1.3 are transposed or reversed, this will not affect the operation ofthe system because the order in which controIled-rectifler-pairs l, 2, and 3 are fired is immaterial to proper functioning of the system. Regardless of which power supply phase is connected to Iin Ll, for example, the voltage on secondary winding S1 will always be in synchronism with the forward voltage on controlled-rectifiers 1. The system can be controlled to provide constant amplitude firing pulses over a full I80 degree firing angle or control range. A 30- degree lag from the leading end of the halfcycle of voltage on secondary winding 81 to the time for dcveloping the earliest firing pulse provides for increased linearity between controlled input and power output. The use of a pulse transformer PLT affords electrical isolation of the input signal circuits from the power supply lines and the rectifier bridge network.

If one of the series-connected controlled-rectifiers fires the 9' arr-mm m L15; ewe-,3 with the te specttve con-trolled recttf sr so tha the firing current pulses are applied to the controlled TECH her: during their positive anode voltage periods regardless of the sequence in which the power supply phomee are conrteeteci to the first mere ttoned input terminals. 5. Ir: :omnation with ts three-phase power etuntrol e roising three input a unis count stable is to the three phasetv my an alternating current rm 3-. ce and a gating tvpe solid state power control flt'merat connected between each input terminal and one side of a load device and a diode connected between the other side of the load device and each such input terminal, and a firing control circuit for each solid state element for producin firing current pulus in controllable :w improvement comprising: l7? rendering said system insensitive to input phase rotation to avoid the necessity of rewrote-mg incorrectly sequenced power connections com prising:

means responsive to the three-phase source voltage for producing three cyclic line-to-neutral control voltages for controlling the firing control circuits; Mu means connecting said control voltage proewwmg means to the input terminals of the power control t att-et'n 'f wet-wen w 'f' ll' n' Mun said system is elimflnv'tted from and rectum nected to a power sous and m a t-Pzgucnce to insure that the firing current pulses are protectunder the time syturhrontzlng control of semi qyclt'c control vottoges in a properly phrased relation with respect to the input power phases applied to said solid Mote elements regardless u] the sequence in which the teen-t torment; em connected to memes Rehretsces fitted The following references, cited by the Emrttiner, are of record in the patented file of this patent or the original patent.

JOHN F. COUCH, Primary Examiner W. M. SHOOP, 12., Assistant Examiner US. Cl. X12. 321-13, 47 

